Electrochemical half-cell with pressure compensation

ABSTRACT

The invention relates to an electrochemical half cell (1) comprising at least one electrode space (3, 15) for receiving an electrolyte (100), a gas space (2), and at least one gas diffusion electrode (14) as anode or cathode separating the gas space (2) and electrode space (3, 15), in which the gas space (2) is subdivided into two or more superimposed gas pockets (2a, 2b), in which gas inflow and gas outflow takes place through separate openings (7) and (12a, 12b, 12c, 12d) and the pressure on the electrolyte side of the electrode (14) is compensated by an opening in the respective gas pocket (2a, 2b) for the electrolyte.

This application is a 371 of PCT/EP97/02689, filed May 26, 1997.

The present invention relates to an electrochemical half cell comprisingat least one electrode space for receiving an electrolyte, a gas space,and at least one gas diffusion electrode as anode or cathode andseparating the gas space and electrode space, in which the gas space issubdivided into two or more superimposed or overlying gas pockets inwhich gas inflow and gas outflow takes place through separate openingsand the pressure on the electrolyte side of the electrode, when comparedwith the pressure on the gas side of the electrode, is compensated by anopening in the gas pockets for the electrolyte.

The operation of electrochemical cells based on gas diffusionelectrodes, for example for use as an oxygen consumption cathode inalkali halide electrolysis, is basically known and is described forexample in U.S. Patent Specification 4,657,651.

The gas diffusion electrode is an open-pore membrane between theelectrolyte and gas space, which has an electrically conducting layerwith catalyst and is intended to enable an electrochemical reaction, forexample reduction of oxygen, to take place at the triple-phase boundarybetween electrolyte, catalyst and reactant gas in the membrane. Theboundary layer is generally held in the membrane by the surface tensionof the electrolyte on the hydrophobic electrode material opposing thehydrostatic pressure of the electrolyte on the membrane. However, inthis connection only a small pressure drop between the gas side andliquid side of the membrane is permissible. If the gas-side pressure istoo high, the gas finally penetrates the membrane and interferes withthe operation of the electrode in this region, and the electrolysisprocess is interrupted. If on the other hand the liquid pressure is toohigh, the triple-phase boundary is forced out from the region containingthe catalyst in the membrane, which likewise interferes with theoperation of the cathode and, if there is a further rise in pressure,causes liquid electrolyte to penetrate the gas space. With verticallyarranged electrodes, as is necessary for example with membraneelectrolysers, in order to be able to remove the desired productchlorine efficiently, this limits the structural height of the gasdiffusion electrodes since otherwise at the top of the electrode gaswill penetrate the cathode space and also at the bottom of the electrodeelectrolyte liquid will penetrate the gas space. The technicallyrealisable structural height is therefore limited to ca. 20-30 cm, whichis unattractive for conventional industrial membrane electrolysis units.

In order to solve the problem of pressure compensation, variousarrangements have been proposed in the prior art.

According to U.S. Patent Specification 4,657,651, pressure compensationbetween the gas space and the electrolyte space on both sides of a gasdiffusion cathode is achieved by subdividing the cathode into individualhorizontal chambers that are individually charged with gas, the gaspressure being regulated by passing the outflowing gas stream in eachcase into vertical chambers in such a way that their depth correspondsto the height of the electrolyte over the respective chamber. Thedisadvantage of this arrangement is the complexity and cost of theequipment, which stands in the way of a technical implementation.Moreover, the pressure in each individual gas chamber has to be adjustedseparately, through respective valves.

The still unpublished German patent application No. P 4 444 114.2describes an electrochemical half cell with a gas diffusion electrode,in which pressure compensation between the gas space and the electrolytespace on both sides of a gas diffusion electrode is achieved bysubdividing the gas space into two or more gas pockets superimposed in acascade-like manner, which are separated from one another and are openat the bottom to the electrolyte so that the pressure in each gas pocketis in equilibrium, via the opening for the electrolyte, with thepressure of the liquid column of the electrolyte in the correspondingpart of the electrode space in front of the gas diffusion electrode, andin which any gas inflow and outflow takes place through the openings forthe electrolyte.

In the publication "Meeting Abstracts 96-1, Abstract No. 949, SpringMeeting May 5-10/1996" of the "Electrochemical Society", a cell with anoxygen diffusion cathode was disclosed for chlor-alkali electrolysis ona laboratory scale, which is a simplified version of the concept ofpressure compensation corresponding to German patent application No. 4444 114.2. FIG. 1 of Abstract No. 949 shows a subdivision of the gasspace in front of the oxygen cathode into two superimposed gas pockets,in which oxygen bubbling from the floor of the cell flows in throughfree openings of the gas pockets. Pressure compensation is achieved bythe vertical subdivision of the gas space into two gas pockets. Themaximum pressure acting on the membrane corresponds to the hydrostaticpressure corresponding to the height of the individual gas pockets.

The design and construction of the aforementioned electrolytic cell havea number of disadvantages that interfere with the operation of thediffusion electrode.

The gas inflow and outflow take place through the same opening of a gaspocket for the electrolyte. The exchange of the reactant gas containedin the respective gas pocket is thereby considerably disturbed since gasfed to the pocket continues to bubble uniformly over the lower edge ofthe gas pocket to the next higher gas pocket. A certain amount of mixingsimply takes place on account of the bursting of the gas bubbles in thegas space behind the electrode. The active outflow and removal ofundesired foreign gases from the gas space of the cell is not possiblein the known electrolysis cell.

Furthermore, the rising gas bubbles can be trapped only to a limitedextent by the equally widely projecting collecting apron of the uppergas pocket. Simple experiments show that most of the gas bubbles flowpast the upper collecting apron of the known electrolysis cell.

The bursting of the gas bubbles at the meniscus of the electrolyte inthe respective gas pocket moreover results in the formation of anundesirable spray mist of electrolyte droplets in the gas space, whichprecipitates on the diffusion electrode and interferes with itsfunctioning.

The object of the invention, which represents an improvement of theprior art, is to develop an electrochemical half cell that has theadvantages of simple pressure compensation but at the same time does nothave the aforementioned disadvantages of the known cells, and that inparticular permits an active ventilation of the gas space behind thediffusion electrode.

This object is achieved according to the invention by an electrochemicalhalf cell, which is the subject of the invention, comprising at leastone electrode space for receiving an electrolyte, a gas space, and atleast one gas diffusion electrode as anode or cathode separating the gasspace and electrode space, in which the gas space is subdivided into twoor more superimposed gas pockets that are separated from one another andeach of which has an opening for the electrolyte, so that the pressurein each gas pocket is in equilibrium, via the opening for theelectrolyte, with the pressure of the liquid column of the electrolytein the corresponding part of the electrode space lying in front of thegas diffusion electrode, characterised in that the gas inflow and thegas outflow of the individual gas pocket are spatially separated fromone another.

In particular, the gas inflow and the gas outflow in a gas pocket arearranged laterally displaced relative to one another so that there is alateral flow of the electrode gas in the gas pocket.

Because of the enforced flow of the electrode gas, gas exchange in therespective gas pocket is improved and the accumulation of undesiredforeign gases, which for example occurs in the known cell arrangement,is avoided.

In addition, the isothermal direct contact of the electrode gas with theelectrolyte means that the gas on the gas side of the diffusionelectrode is always saturatedly wet and a "crystallising out" ofelectrolyte substances, especially in the membrane structure of thediffusion electrode, is thereby avoided.

This in turns prevents irreversible damage to the electrode by crystalsof electrolyte.

In a particular embodiment of the invention the gas pockets are formedas allround closed chambers, one of whose boundary walls is the gasdiffusion electrode and which have at a side end the gas feed or inflowfor electrode gas. Excess electrode gas is removed at another side endof the gas pockets through a dip tube dipping into thepressure-compensating, stagnant electrolyte liquid. Because of thisarrangement of gas feed on one side and removal of excess gas on theother side of the gas pocket, there is an active forced lateral flow ofelectrode gas through the gas pockets. With this cell arrangementpressure compensation between the gas space and electrode space takesplace via the gas outflow. An advantage compared with the simplearrangement is that through the above design and construction of thehalf cell, an active gas exchange takes place in the gas pocket, whichcan be controlled by altering the amount of excess gas. This preventsany possible enrichment of interfering foreign gases in the gas pockets.Furthermore, it is thus possible to use less pure electrode gas oralso--depending on the intended use of the half cell--to remove possiblyformed product gas from the electrode reaction.

In a preferred variant of the invention the forced lateral ventilationof the half cell with electrode gas is achieved by alternately arrangingthe gas inflow and gas outflow from a gas pocket to the nexthigher-lying gas pocket so that the gas outflow of a gas pocket islocated underneath the gas inflow of the next higher-lying gas pocket.

The gas pockets can have collecting aprons that are secured to the rearwall of the gas pocket. In the simplest form a guide plate or baffle forcollecting the gas bubbles ascending from a lower gas pocket outlet isarranged vertically on the rear wall of the gas pocket and projectslaterally over the outlet of the gas pocket outflow located thereunder.

Compared with an arrangement known from the prior art with superimposedgas pockets whose gas collecting aprons form part of the rear wall ofthe gas pockets, the above arrangement has the advantage that the halfcell according to the invention can be designed and constructed to bemuch narrower since the gas collecting apron of the known arrangementprojects a long way back from the gas diffusion electrode, whereas thevertical height of the laterally arranged gas collecting aprons, viewedfrom the electrode surface, can be kept small.

In a variant of the invention, instead of the lateral gas collectingaprons, a bubble channel can be used as gas inflow for the gas pockets,which projects downwardly and opens into the electrolyte. The bubblechannel is for example arranged above the outlet of the gas outflow of alower gas pocket, so that exiting electrode gas bubbles up into thebubble channel. The bubble channel can have a laterally widened bubbletrap at its lower end.

In another variant the lower outlet is formed as a U-tube, one of whosearms projects a bit more widely and opens out into the opening of thebubble channel.

The aforementioned variants permit a transfer of electrode gas from gaspocket to gas pocket, so that the gas bubbles of electrode gas arrivingat the meniscus of the electrolyte fluid still burst in a region outsidethe actual gas pocket. This avoids the rear side of the diffusionelectrode being sprayed with electrolyte. This beneficial feature is inparticular also achieved if the bubble channel or the gas collectingapron is laterally displaced, for example moved to the middle of thehalf cell, opposite the opening to the chamber of the gas pocket. Inthis case the gas collected in the bubble channel or gas collectingapron is passed laterally through a pipe to the opening.

This produces a particularly efficient cleaning and removal of spraymist. Any "pulsations" caused by the bursting of gas bubbles are nottransmitted to the gas pocket.

The inlet of the gas feed to each gas pocket can be designeddifferently. Besides using a simple opening, a plurality of superimposedopenings or one or more inlet slits, extending at most over the heightof the gas pocket, may be provided in order for example to supply therear side of the electrode with fresh electrode gas over its wholeheight.

In order to trap spray mist the openings can be covered with baffles asblocking means opposite the electrode wall, on which electrolytepossibly entrained with the gas stream settles out and can flow back.

A further variant of the pressure-compensated half cell according to theinvention has, instead of an electrode gas flow running from a gaspocket to the overlying gas pocket, an individual feed for theindividual superimposed gas pockets by means of gas feed lines which,depending on the circumstances, are equipped with their own control andshut-off valves.

The pressure compensation of the individual gas pockets occurs in eachcase via the gas outflow of the gas pockets, which dips openly into theelectrolyte.

This preferred arrangement is advantageously employed if for exampleharmful gases are formed in the electrode reaction that inhibit thisreaction, and which can accumulate in the case of a meandering gas flowfrom the lowest gas pocket up to the uppermost gas pocket.

This arrangement also enables part of the gas diffusion electrodesurface to be "disconnected" by flushing the selected gas pocket with aninert gas (for example a noble gas). In this way it is possible tocontrol and monitor the individual performance of the "disconnected"electrode surface when the half cell is operating.

In all the aforementioned embodiments of the invention the gas pressurein the region of each gas pocket corresponds to the liquid column of therespective outlet opening or of the lower edge of the bubble meniscus upto the upper edge of the liquid column between the gas pocket cascadeand the rear wall of the electrolysis cell. This pressure is compensatedby the liquid column in the electrode chamber, the equilibrium lying atthe respective gas outlet opening or lower edge of the aforementionedbubble meniscus when both chambers are filled to the same height (e.g.when both chambers are hydraulically connected to one another). Since ahomogeneous pressure prevails in the relevant gas pocket, on average aslight excess pressure exists on the gas side, which is also desirablein terms of optimum functioning, for example of catalytic oxygenreduction.

If in a further preferred variant of the half cell according to theinvention the electrode space and the rear wall electrolyte space arehydraulically separated, then the respective differential pressure,which is of course the same for all chambers, can be specificallyadjusted by different filling levels or outflow heights in bothchambers.

For example, a controllable excess pressure can be adjusted by separategas removal through a pipe passing upwardly to the gas discharge lineand an optionally provided electrolyte receiver arranged thereabove,which is then for all gas pockets at the same height relative to theelectrode space.

If on the other hand outflow of electrolyte from the cell preferablytakes place downwardly through a stand pipe or also optionally at a sidewall of the cell, it is directly possible to remove electrolyte andexcess gas together by allowing the electrolyte from the electrode spaceto flow exclusively upwardly over the gas pocket electrode into the rearelectrolyte space, from where it flows downwardly from the cell throughthe stand pipe together with the excess oxygen, or in the case of alateral outflow, also to the side. Different heights of the stand pipeproduce different differential pressures, this time the liquid pressurebeing greater than the gas pressure, which is of advantage particularlywhen the whole surface of the pocket-like gas diffusion electrodes is incontact with the current distribution grid. If desired, retaining andclamping devices for the electrode can then in fact be omitted. In aperfectly similar way to the joint removal of electrolyte and excess gasvia the stand pipe, this removal can also be effected through adischarge pipe arranged laterally on the half cell, the separation ofgas and electrolyte taking place for example in a collector as well asthe cell. Furthermore, in this way the liquid pressure can be adjustedhigher than the gas pressure over the gas diffusion electrode.

The half cell according to the invention can be expanded by anappropriate number of gas pockets to any arbitrary and technicallyfeasible size. Since the amount of gas (e.g. oxygen) required fortypical electrolysis loads is for example 0.7 to 1 standard cubic meterper square meter of cathode surface per hour, the necessary gastransport can be achieved without any problem by choosing a suitablesize for the bubble openings, as hydraulic tests have demonstrated.

The retention and electrical contact of the gas diffusion electrode,especially in half cells of membrane electrolysis devices, is basicallyknown. When using several electrode segments as gas diffusion electrode,the gas diffusion electrode segments are held in a gastight manner withrespect to one another and to the electrode space.

The retaining elements for the gas diffusion electrode may be designedfor example as clamping strips or suitably encased magnetic strips,which above all serve as installation aids.

In the case of electrolysis cells with an interposed ion exchangemembrane, after assembly and installation the retaining elements may besupported via the ion exchange membrane against the counterelectrodestructure lying behind the latter and thus provide for a sufficientcompression against the gas diffusion electrode, which is thereby alsobrought into electrical contact.

The retaining elements may in the case of an electrolysis cell havenotches aligned in the flow direction, on the side of the cell facingthe ion exchange membrane, which notches permit a homogeneousflowthrough from compartment to compartment of the electrode space alsowhen the electrolysis cell is braced or clamped.

By means of a suitable distribution of the inflow notches, for examplewith an increasing number of notches from the bottom upwards, an extrahydraulic pressure loading on the lower electrode compartments isavoided.

In a particularly advantageous arrangement of the invention an elasticspacer fills up the narrow electrode space, which not only acts as aspacer and turbulence generator for the electrolyte stream, but can alsobe mounted over the aforementioned retaining elements and clampedtogether with the latter, thereby forming a further elastic componentfor compressing and sealing the gas diffusion electrode.

In another embodiment the spacing between the gas diffusion electrodeand membrane is ensured by sheathed wires, which are passed verticallythrough the individual compartments and are clamped in the notches ofthe retaining elements.

The retention of the gas diffusion electrode segments can also beeffected by means of a T-shaped retaining device, whose longer armterminates in appropriate sections in clips that are inserted in such away through the retaining structure that the latter can be tightenedfrom the back, for example by means of clamping wedges driven throughsuitably arranged bores. The gas diffusion electrode and optionally aseal are thus pressed by means of the short arm of the T-shapedretaining device in such a way against the retaining structure, which isdesigned as a low impedance power supply, that gastightness as well as agood current contact are ensured.

Current can be supplied to the gas diffusion electrode by arrangementsknown per se. Preferably current is fed through the retaining device ofthe gas diffusion electrode, which in turn is connected in alow-impedance manner together with the rear side of the electrolysiscell to an external power supply, an additional metallic grid structurebeing mounted on the retaining device, the gas diffusion electrodecontacting the grid structure on the gas side or electrolyte sidedepending on the differential pressure between the electrolyte side andthe gas side, and the grid structure providing short current paths. Inthe case of a gas diffusion electrode and integrated metallic grid, theseparate metallic grid structure on the retaining device can optionallybe omitted if the diffusion electrode can be supported in the directionof the gas space by another simple abutment.

The current may also preferably be supplied through a low impedanceconnection to the rear of the half cell.

An advantageous embodiment of the half cell according to the inventionis characterised in that the whole structure of the gas pocket electrodeis designed so that it can be removed from the half cell, for example anelectrolysis half cell.

The half cell according to the invention can in principle be used in allelectrochemical processes in which a gas diffusion electrode is operatedin direct contact with a liquid electrolyte. Examples of the use of thehalf cell according to the invention include the following:

sodium dichromate electrolysis; a hydrogen consuming anode is used forexample; hydrogen production at the cathode can be replaced by oxygenreduction at an oxygen consuming cathode;

hydrogen peroxide production by reducing oxygen at a gas diffusioncathode;

use in alkaline fuel cells, which are employed for example toconcentrate sodium hydroxide solutions. The fuel cells can be operatedwith half cells corresponding to the invention, connected up as theanode for hydrogen conversion, and with half cells connected up as thecathode for oxygen reduction.

By means of the half cell according to the invention the conventionalmembrane electrolysers available on the market for electrolysing alkalihalide solutions can in principle be converted to an energy-savingoperation with for example oxygen consuming cathodes, as long as theyhave a sufficiently deep cathode chamber.

This applies in particular also to cell types with a vertical ribstructure or vertical or horizontal internal structural ribs.

In principle all known types of gas diffusion electrodes can be used inconjunction with the half cell according to the invention, for exampletypes with integrated metallic supporting grids or current distributiongrids, or electrodes installed on carbon blocks or other conductingstructures.

Further preferred embodiments of the half cell according to theinvention are disclosed in the sub-claims.

The invention will now be illustrated in more detail with the aid of thefigures, which however do not restrict the invention.

In the figures:

FIG. 1 is a variant of the half cell according to the invention, withbubble channels designed as oxygen consuming cathode, shown incross-section parallel to the diffusion electrode surface.

FIG. 2 is a cross-section through the half cell according to FIG. 1corresponding to the line 2-2 in FIG. 1.

FIG. 3 is a cross-section through the half cell according to FIG. 1corresponding to the line 3-3 in FIG. 1.

FIG. 4 is a detail of a variant of the half cell according to theinvention according to FIG. 1, with separate gas feed.

FIG. 5 is a cross-section of a half cell according to the invention withlaterally extended gas collecting aprons.

FIG. 6 is a cross-section of an example of the half cell according tothe invention with direct gas feed to the individual gas pockets.

FIG. 7 is a section corresponding to the line 7-7 in FIG. 6 through thehalf cell according to FIG. 6.

FIG. 7a is an enlarged detail of FIG. 7.

FIG. 8 is a section corresponding to the line 8-8 in FIG. 6 through thehalf cell according to FIG. 6.

FIG. 9 is a cross-section of a variant of the half cell according toFIG. 6 with gas feed to the gas pockets through plug-in pipes.

FIG. 10 is a section corresponding to the line 10-10 in FIG. 9 throughthe half cell according to FIG. 9.

EXAMPLES Example 1

An electrochemical half cell connected up as an oxygen consumptioncathode has the following basic design and construction (see FIG. 2).

The half cell 1 is separated from another half cell (not shown) by amembrane 13. Electrolyte 100 (in this case aqueous NaOH) is addedthrough a feed line 9 to the electrode space 20 and flows between themembrane 13 and a gas diffusion electrode 14 through the electrolyte gap15 to the collecting chamber 5. The electrode 14 is connected via a lowimpedance electrical contact (not shown) to an external power source.

The gas space 2 behind the diffusion electrode 14 is subdivided intosuperimposed gas pockets 2a, 2b, 2c and 2d. The rear space 3 behind thegas pockets 2a-2d contains electrolyte 100 that is in pressureequilibrium via the collecting chamber 5 with the electrolyte 100 in theelectrolyte gap 15.

The various examples that follow basically differ from one another bythe particular arrangement of the gas inflow and gas outflow of the gaspockets 2a-2d.

After the lowermost gas pocket 2a has been directly fed with gas, theexcess gas is led through a dip tube 12a into a bubble channel 3a, asillustrated in FIG. 1. The bubble channel 3a is open at the top and isthus in communication with the pressure-compensating electrolyte 100 ofthe rear space 3. At the side and top the bubble channel 3a is closed asfar as an opening 7 in the next higher gas pocket 2b. The rear side ofthe bubble channel 3a is formed either by the rear wall of theelectrochemical half cell 1 (see FIG. 2: section 2-2) or by anindependent closure wall (not shown). The latter structure enables forexample the inserted gas pocket to be installed and dismantledindependently. The ascending bubbles separate from the electrolyte inthe upper region of the bubble channel 3a at the level of the meniscus,which is set by the end of the dip tube 12b to remove excess gas on theother side of the next higher gas pocket 2b, taking into account thebubble effects in the bubble channel 3a. The pressure compensation inthe pressure-compensating back liquid, which in this case is theelectrolyte 100 actively participating in the reaction, is effectedthrough bores 19 in the rear structural elements 6a-6d (see FIG. 3,section 3-3). Active flow through the electrolyte gap 15 between themembrane 13 and gas diffusion electrode 14 is ensured by a baffle 18between the rear space 3 and electrolyte distribution chamber 20,whereas in the upper collecting chamber 5 the electrolyte gas 15 andrear space 3 are connected to one another for the pressure compensation.

Example 2

In the particularly advantageous variant of the half cell 1 asillustrated in FIG. 1 and in more detail in FIG. 4, the bubble channels3a-3d have no direct opening into the respective gas pockets 2a, 2b, 2c,2d, but instead have a lateral connection 21 in the upper region of theaccumulating gas bubbles in the region 22, which only opens downwardly,in addition to the bubble channel 3a-3d. The quiescent gas, freed fromgas bubbles burst by the spray, is led from the region 22 through a bore23 into the next gas pocket 2b-2d, as illustrated in FIG. 4. Thehorizontal arrangement of the bubble channels 3a-3d is arbitrary and isgoverned only by the structural boundary conditions of the respectiveelectrochemical half cell. The decisive factor for the active flowthrough the gas pockets 2a, 2b, 2c and 2d is simply the, in each case,lateral arrangement of the gas inflow and outflow opening of each gaspocket 2a-2d, which in the case of the gas outflow is optionallyrealised by an internally situated and laterally displaced gascollecting tube (not shown) on the side opposite to the gas inflow.

In both cases the further addition of the electrode gas takes place in acascade-like manner through similar sub-assemblies from lower gaspockets 2a-2c to the next higher gas pocket 2b-2d up to the discharge ofthe unconsumed electrode gas, which for example is removed together withthe electrolyte through a pipe 11.

This variant is particularly suitable for electrolysers with verticallyarranged structures.

Example 3

A further alternative way of achieving pressure compensation ischaracterised by the following elements: after the lowermost gas pocket2a has been charged with gas as described in Example 1 the excess gasleaves the gas pocket 2a, which is otherwise closed on all sides,through an opening 30a, corresponding to FIG. 5, and collects in theside apron 31a, which in turn is separately closed on the rear side orforms a gastight seal with the rear wall of the electrochemical halfcell. The excess electrode gas bubbles through the side apron 31a, whichis open at the bottom and is in communication with thepressure-compensating liquid, into the side apron 32a, which is of asimilar construction to the apron 31a. The side apron 32a is extendedlaterally to a sufficient extent so that the ascending gas bubbles fromthe gas pocket 31a are more securely trapped. The electrode gas collectshere and enters the next higher gas pocket 2b through the opening 30b.The excess gas leaves the gas pocket 2b through the opening 30c, whereit collects in the side apron 31b, overflows, and is collected by theside apron 32b. The process is repeated in a cascade-like manner untilthe gas leaves the cell, in a similar manner to that described inExample 1. The essential feature is that the region outside the sideaprons and behind the gas pockets 2a-2d is constantly filled with thepressure-compensating liquid. This arrangement enables a particularlyflat half cell to be constructed.

Example 4

For specific applications, for example in cases in which an accumulationof harmful or toxic gases, which is inevitable in the cascade-typearrangement of the gas pockets 2a-2d, especially in the upper gaspockets, must be avoided, a direct charging of the respective gas pocket2a-2d with fresh electrode gas may be necessary. In the arrangementaccording to FIG. 6 the bubble channels 42a to 42d used for the gasoutflow are designed in a similar way to those in Example 1. However, incontrast to the situation in Example 1 the respective gas inflowchannels 40a, 40b, 40c and 40d are extended directly up to therespective gas pockets and lead through corresponding openings, as shownfor example at 41c in section 7-7 in FIG. 7, into the gas pocket 2c. Itis essential that the bubble channels 40a-40d are open at the bottom andare in direct communication with the pressure-compensating liquid(electrolyte 100). The bubble channels are fed with gas through thedistribution tube 44, which feeds in each case through a nozzle 45 intothe relevant bubble channel 40a-40d. Isobaric conditions exist at therespective nozzle 45 on account of the direct communication with thepressure-compensating liquid, which results in a uniform feed to therespective bubble channels. In order to prevent electrolyte 100 flowingback into the nozzles 45, the latter are covered with sealing cone-likecaps 46 (see section A-A' in FIG. 7a). The controlled release of bubblesoccurs through slits 47 at the lower end of the caps 46. The electrodegas flows through the gas pocket and leaves the pocket, for examplepocket 2c, at the opposite end through the dip tube 42c, as isillustrated in section 8-8 in FIG. 8 together with the pressurecompensation. The gaps 19 in the rear structural elements 6a-6e in turnensure a free ascent of the bubbles and the necessary pressurecompensation.

Example 5

A further variant of the half cell according to the invention isillustrated in FIG. 9: here the gas is fed into the individual gaspockets 2a-2d through direct insert tubes 50a, 50b, 50c and 50d. Inorder to be able to ensure a uniform feed of the gas pockets 2a-2d inthis case, the individual pipes 50a-50d must be throttled with respect.to the feed pipe 51, for example by means of throttle valves 52a-52d.The adjustment may however be made in a one-off manner or in the form ofa permanent installation. As illustrated in section 10-10 in FIG. 10,the unconsumed electrode gas leaves the gas pocket 2c through the diptube 42c, in exactly the same way as described in the preceding example,after flowing through the said gas pocket 2c. In both cases the rearstructural elements 6a-6e have gaps in order to ensure that gas bubblesare freely transported away and that pressure compensation can takeplace.

In particular, the last variant permits a particularly flat half cellconstruction.

What is claimed is:
 1. Electrochemical half cell (1), comprising atleast one electrode space (3, 15) for receiving an electrolyte (100), agas space (2), and at least one gas diffusion electrode (14) as anode orcathode and separating the gas space (2) and electrode space (3, 15), inwhich the gas space (2) is subdivided into two or more superimposed gaspockets (2a, 2b, 2c, 2d) that are separated from one another and have anopening (7) for the electrolyte (100) so that the pressure on each gaspocket (2a, 2b, 2c, 2d) is in equilibrium via an opening (7) for theelectrolyte (100) with the pressure of the liquid column of theelectrolyte (100) in the corresponding part (15) of the electrode space(3, 15) in front of the gas diffusion electrode (14), wherein the gasinflow (7) and the gas outflow (12a, 12b, 12c, 12d) of the individualgas pockets (2a, 2b, 2c, 2d) are spatially separated from one anotherand the gas inflow (7) and the gas outflow (12a, 12b, 12c, 12d) arearranged laterally displaced from one another in the individual gaspockets (2a, 2b, 2c, 2d) so that there is a lateral flow of theelectrode gas in the gas pocket (2a, 2b, 2c, 2d).
 2. Half cell accordingto claim 1, wherein characterised in that the superimposed gas pockets(2a, 2b, 2c, 2d) are connected to one another in a cascade-likearrangement by connecting the gas outflow (12a, 12b, 12c, 12d) of alower gas pocket (2a, 2b, 2c) to the gas inflow (3a, 3b, 3c, 3d) of anoverlying gas pocket (2b, 2c, 2d).
 3. Half cell according to claim 1,wherein characterised in that the gas pockets (2a, 2b, 2c, 2d) each havea direct gas inflow.
 4. Half cell according to claim 3, wherein the gaspockets (2a, 2b, 2c, 2d) have insert pipes (50a, 50b, 50c, 50d) fordirect gas inflow, which are optionally provided in addition in eachcase with control valves (52a, 52b, 52c, 52d) to regulate the gas flowinto the gas pockets (2a, 2b, 2c, 2d).
 5. Half cell according to claim1, wherein characterised in that the gas pockets 2a, 2b, 2c, 2d) have adip tube (42a, 42b, 42c, 42d) for removing excess gas from the gaspocket (2a, 2b, 2c, 2d).
 6. Half cell according to claim 1, whereincharacterised in that the gas pockets (2a, 2b, 2c, 2d) have bubblechannels (3a, 3b, 3c, 3d) for receiving inflowing electrode gas, whichare directly or indirectly connected to the opening (7).
 7. Half cellaccording to claim 1, wherein characterised in that the gas pockets (2a,2b, 2c, 2d) have lateral gas collecting aprons (32a, 32b, 32c, 32d) asgas inflows and/or lateral aprons (31a, 31b, 31c, 31d) as gas outflows.8. Half cell according to claim 1, wherein the gas pockets (2a, 2b, 2c,2d) have open bubble channels (40a, 40b, 40c, 40d) for the electrolyte(100) for a direct independent gas feed.
 9. Half cell according to claim8, wherein the gas feed to the bubble channels (40a, 40b, 40c, 40d)takes place via throttled nozzles (45), which are optionally covered bysealing cones (46) having side slits.
 10. Half cell according to claim1, wherein characterised in that the gas inflow (7) and the gas outflows(12a, 12b, 12c, 12d) of superimposed gas pockets (2a, 2b, 2c, 2d) lie onalternate sides or in each case on the same side.
 11. Half cellaccording to claim 1, wherein the electrolyte gap (15) and electrolyterear space (3) are hydraulically separable to adiust the differentialpressure between the regions in front of and behind the gas diffusionelectrode (14).
 12. Half cell according to claim 1, comprising aconnecting pipe (10) through which the gas inflow into the lowermost gaspocket (2a) takes place coaxially together with the electrolyte inflow(9) into the electrode space (3, 15) and/or the discharge of the excessgas together with the electrolyte takes place at the top through anoutflow pipe (11).
 13. Half cell according to claim 1, wherein theelectrolyte gap (15) is hydraulically connected at the top to theelectrolyte rear space (3) behind the gas pockets (2a, 2b, 2c, 2d),overflows into the latter, and the discharge of the excess gas togetherwith the electrolyte (22) occurs downwardly through a stand pipe in theregion behind the gas pockets (2a, 2b, 2c, 2d) or occurs sidewaysthrough a laterally arranged pipe with a gas-liquid separator situatedat the same height.
 14. Half cell according to claim 13, wherein theheight of the stand pipe in the electrolyte rear space 3 behind gaspockets (2a, 2b, 2c, 2d), the height of the laterally arranged pipe orboth, are adjustable to admust the liquid level of the electrolyte (100)differently relative to the level of the electrolyte (100) in theelectrolyte gap (15) to vary the differential pressure between the gasspace (2) and electrolyte gap (15) for all gas pockets (2a, 2b, 2c, 2d).